I bought the cheapest Brompton in the UK* What have I bought?

(* based on my limited check of eBay, Gumtree and Facebook Marketplace)

Won an eBay auction on this old Brompton at the weekend. The frame number is 24XX so I reckon it is 1990. It has been stored at a second hand shop for years apparently and was absolutely filthy.
Spend a couple of hours stripping back the dirtiest parts and degreasing everything. Inspected the frame and all looks ok (eg no cracking due to seat post tightening etc) but the paintwork is best described as "Willem Defoe". Rugged, aged, not exactly handsome with undoubtedly a few storeis to tell! There is some surface rust which comes off pretty easily particularly at the base of the seat post where the frame number is and at the bottom of the front forks. It will need a new paintjob though.
It has a 3 speed Sturmey Archer hub marked 89-09 which tallies with the frame / date.
Needs a new chain, new brake and gear cables (note the rear isn't yet attached) – the outers actually look ok. I have taken off the bottle dynamo and front light – the rear light bolt is super tight so I shall tackle that another day. I have fitted a Charge saddle that I already had lying about.
Miraculously the tyres have held air overnight.
Note – this is an n+1 bike – I don't need it, but I wanted a Brompton, therefore wouldn't justify buying a new or better used version. It will be used for local short trips – no plans for bike-packing etc yet!
This is my first Brommie – so I need to ride it a bit before I invest much more into it.
Do you think I should try and keep this original to retain some value given its age? Or do a kind of rolling resto-mod – I am thinking dual pivot brake and lever upgrade, new seatpost, new tyres?
If it actually has some value as an authentic restoration for someone, I could perhaps sell it on, make a small profit to then buy another newer used one?
BTW – I am a pretty competent home bike machanic and will tackle most jobs myself where tools etc allow.